A density functional study of the effect of reduction on the geometry and electron affinity of hydrogen bonded 1,4-benzoquinone. Implications for quinone reduction and protonation in photosynthetic reaction centres

Hybrid density functional studies using the B3LYP functional show that the two electron reduction of 1,4-benzoquinone (BQ) to the dianion (BQ2−) via the semiquinone radical anion (BQ−) form is accompanied by a gradual change in the internal geometry of the quinone from quinonoid to benzenoid form. The hydrogen bonds formed to neighbouring proton donors are progressively shortened. These changes contribute to the ultimate transfer of the hydrogen bonded protons from the hydrogen bonded proton donors leading to the formation of the quinol form. The reduced forms are progressively stabilised relative to the oxidised from with increasing hydrogen bonding interactions. Similar changes can be expected to play a crucial role in the reduction and protonation of the Qb site of photosynthetic reaction centres in bacteria and higher plants.